The invention generally relates to devices of variable transmission to electromagnetic radiation, more particularly the invention relates to materials with enhanced UV stability for use in these devices.
Devices of variable transmittance to electromagnetic radiation have found application as the variable transmittance element in variable transmittance light filters, variable reflectance minors and display devices, which employ such light filters or mirrors to display information. Architectural windows, skylights, automotive windows and sunroofs are also included in these light filters. Variable reflectance mirrors have become useful as anti-glare rearview mirrors in automobiles.
Devices of variable transmittance to electromagnetic radiation wherein the electromagnetic radiation is attenuated by electrochromic means are described, for example by Chang, "Electrochromic and Electrochemichromic Materials and Phenomena" in Non-emissive Electro-optic Displays, A. Kmetz and K. vonWillisen, eds. Pergamon Press New York, N.Y. 1976, pp. 155-196(1976).
Numerous electrochromic devices are known in the art. See, e.g. Manos, U.S. Pat. No. 3,451,741; Bredfelt et al, U.S. Pat. No. 4,0900,782; Shattuck and Sincerbox, U.S. Pat. No. 4,093,358; Cleacak et al., U.S. Pat. No. 4,139,276; Kissa et al., U.S. Pat. No. 3,453,038; Rogers U.S. Pat. Nos. 3,652,149, 3,774,988, and 3,873,185; and Jones et al., U.S. Pat. Nos. 3,282,157, 3,282,158, 3,282,160 and 3,283,656.
In addition to these devices, there are commercially available electrochromic devices and associated circuitry, see e.g. Byker et al., U.S. Pat. Nos. 4,902,108, 5,128,799, 5,202,787, 5,280,380, 5282,077, 5,294,376 and 5,336,448; Bechtel et al., Canadian Pat No. 1,300,945, U.S. Pat. Nos. 5,204,778 and 5,451,822; Bauer et al., U.S. Pat. No. 5,434,407 and Tonar, U.S. Pat. No. 5,448,397. Each of these patents is commonly assigned with the present invention and the disclosures of each including the references contained therein are hereby incorporated in their entirety by reference.
It is desirable to use reversibly variable transmittance filters in architectural windows, skylights and in windows and sunroofs for automobiles in order to allow for user control of the transmittance through the same. For example, it is desirable to reduce the amount of sunlight, and hence glare and heat, transmitted through a window at specific times of the day and year, while allowing for higher transmittance at other times. In addition, fading of materials within the building or vehicle can be reduced by reducing the sunlight transmitted through a window.
Electrochromic devices typically include a structure similar to that shown in FIG. 1. Specifically they include a transparent substrate 12 and a second substrate 14 which are arranged in a parallel, spaced-apart relationship. The electrochromic devices also typically include an electrically conducting layer forming electrode 16 on the interior surface of substrate 12. Transparent conductive coatings or a metallic mesh or grid can be used for this electrically conducting layer. A second electrode 18 is provided on the interior surface of substrate 14. A seal 20 is provided to secure the substrates together and to provide a chamber 22 between the substrates in which an electrochromic medium 24 is disposed. Electrical connection can be made to the electrodes 16 and 18 by clips (26 and 28) conductive pastes, solders, etc. The application of a sufficient potential to the electrodes causes the electrochromic medium to reversibly modulate electromagnetic radiation incident through device.
The electrochromic medium 24 generally includes electrochromic anodic and cathodic materials that upon activation, due to the application of an electronic voltage or potential, exhibit a change in absorbance at one or more wavelengths of the electromagnetic spectrum. Commonly electrochromic devices have been solution-phase, electrodeposition or solid metal oxide type devices.
Heretofore, electrochromic devices have not found wide acceptance as light filters in windows, skylights or automobile windows or sunroofs due in part to the effects of sunlight, specifically UV radiation (400-200 nm) on the materials within the electrochromic medium. In the medium 24 as the anodic and cathodic materials are oxidized and reduced, they undergo changes in their electronic configuration. These electronic configuration changes result in the changes in absorbance and also result in the possibility of new, and possibly detrimental chemical reactions taking place. If one or more of these reactions leads to a product that cannot be returned to the inactivated form of the anodic or cathodic material and/or results in a colored product, the medium is said to undergo degradation. This degradation can lead to residual tints or colors in the device in its unactivated state or loss of coloration capacity when the device is activated. These reactions can be initiated by photons or by the increase in temperature a device endures when it is activated in sunlight.
Attempts to overcome the effect of sunlight on the electrochromic medium have included the addition to the electrochromic medium of certain UV absorbing materials, such as stabilizers, or solvents which absorb in the UV, to the medium, see Varaprasad et al., U.S. Pat. Nos. 5,140,455, 5,340,503 and 5,725,809. Alternatively placing an UV blocking barrier in front of the electrochromic medium has been described, see e.g. Lynam U.S. Pat. No. 5,864,419. These two methods have also been combined see e.g. Lynam U.S. Pat. No. 5,073,012. These attempts, while having somewhat improved the stability of the electrochromic medium, still have not lead to an electrochromic medium with acceptable daylight stability, and therefore have yet to lead to a commercially successful electrochromic device useful as a window, skylight or automotive window glazing.
Thus the need still exists for an electrochromic medium that has enhanced stability. In addition, materials with enhanced stability will be useful in making an electrochromic medium with enhanced stability.